1. Field of the Invention
The present invention relates to a color image forming system, and more particularly, to a color image forming system having a multi-path method using electrophotography.
2. Description of the Related Art
Conventional color image forming systems using electrophotography radiate light onto a photosensitive body charged to a predetermined potential, form an electrostatic latent image, develop the electrostatic latent image with toner having a predetermined color using a developing unit, fuse the developed electrostatic latent image onto paper and form a color image. The colors of toner used in color image forming systems include yellow (Y), magenta (M), cyan (C), and black (K). Thus, four developing units, each to develop only one of the four colors of toner, are necessary.
Methods of forming a color image include a single path method using four exposing units and photosensitive bodies, and a multi-path method using one exposure unit and one photosensitive body.
FIG. 1 illustrates a color image forming system using the single-path method. As shown in FIG. 1, the color image forming system using the single-path method includes photosensitive bodies (drums) 120C, 120M, 120Y, and 120K, exposing units 110C, 110M, 110Y, and 110K, and developing units 130C, 130M, 130Y, and 130K provided for toner having four colors such as cyan (C), magenta (M), yellow (Y), and black (K). Each of the photosensitive drums 120C, 120M, 120Y, and 120K is placed near a transfer belt 140. The transfer belt 140 is driven by two driving rollers 150 at a predetermined speed. The transfer belt 140 is placed between one of the two driving rollers 150 and a transfer roller 160, and a piece of paper S is transferred between the transfer roller 160 and the transfer belt 140.
The method of forming a color image using the above structure is as follows. First, light which corresponds to image information of cyan (C) color, is radiated by the exposing unit 110C onto the photosensitive drum 120C, thereby forming an electrostatic latent image. Then, a toner of cyan (C) color stored in the developing unit 130C is attached to the electrostatic latent image, a toner image of cyan (C) color is formed on the photosensitive drum 120C, and the toner image is transferred onto the transfer belt 140. At a predetermined amount of time after exposure of the photosensitive drum 120C with the light from the exposing unit 110C corresponding to image information of the cyan (C) color, the exposing unit 110M radiates light which corresponds to image information of magenta (M) color, onto the photosensitive drum 120M, and forms an electrostatic latent image. Then, a toner of magenta (M) color stored in the developing unit 130M is attached to the electrostatic latent image, a toner image of magenta (M) color is developed onto the photosensitive drum 120M, and the toner image is transferred onto the transfer belt 140. In this case, the starting time of exposure by each of the exposing units 110C and 110M is adjusted such that the toner image of cyan (C) color and the toner image of magenta (M) color that are transferred onto the transfer belt 140 are precisely overlapped on the transfer belt 140. Subsequently and in the exact same way, toner images of yellow (Y) and black (K) colors are also transferred onto the transfer belt 140, thereby forming color toner images. These color toner images are transferred onto the piece of paper S passing between the transfer belt 140 and the transfer roller 160, and are fused by a fusing unit 170 onto the piece of paper S by heat and pressure, thereby forming a complete color image.
In a color image forming system using the single-path method, a complete color image may also be formed by rotating the transfer belt 140 once. Alternately, a black-and-white image is formed by rotating the transfer belt 140 once. That is, the time required to color print is the same as the time required for black-and-white printing. Thus, the single-path method is widely used in high-speed color image forming systems.
However, if an exposure starting time is not precisely adjusted in consideration of the relative positions of the exposing units 110 and the relative positions of the photosensitive drums 120, the toner images of each color are not precisely overlapped on the transfer belt 140. Thus, a good quality color image cannot be obtained. In addition, since the four exposing units 110 and the four photosensitive drums 120 are necessary, the price of the color image forming system becomes higher.
Another type of color image forming system, which avoids the above problems, operates in a comparatively low-speed region, adopts one photosensitive drum and one exposing unit, and employs a multi-path method of forming a color image by repeating exposure, development, and transfer operations for each color. Multi-path methods include a rotary method and a slider method which differ in the arrangement of the developing units for each color and the way in which an individual developing unit is activated.
FIG. 2 illustrates a color image forming system using the rotary method. As shown in FIG. 2, the color image forming system using the rotary method includes one photosensitive drum 220, one exposing unit 210 which radiates light onto the photosensitive drum 220, a transfer belt 240 placed near the photosensitive drum 220, and a rotating turret 280. Four developing units 230C, 230M, 230Y, and 230K are arranged to each occupy one quarter (90 degrees) of the turret 280. As the turret 280 rotates, the four developing units 230C, 230M, 230Y, and 230K sequentially arrive directly opposite to the photosensitive drum 220. The length of the transfer belt 240 is equal to or greater than a maximum length of the piece of paper S used in the color image forming system.
The operation of a color image forming system having the above structure is as follows. If the turret 280 rotates so that the cyan (C) developing unit 230C is opposite to the photosensitive drum 220, light corresponding to image information of cyan (C) color is radiated by the exposing unit 210 onto the photosensitive drum 220, thereby forming an electrostatic latent image. Then, the toner of cyan (C) color stored in the developing unit 230C is attached to the electrostatic latent image, a toner image of cyan (C) color is formed on the photosensitive drum 220, and the toner image is transferred onto the transfer belt 240.
After the formation of the toner image of cyan (C) color on the transfer belt 240 is completed, the turret 280 rotates by 90 degrees so that the magenta (M) developing unit 230M is opposite to the photosensitive drum 220, light corresponding to image information of magenta (M) color is radiated by the exposing unit 210 onto the photosensitive drum 220, thereby forming an electrostatic latent image. Then, the toner of magenta (M) color stored in the developing unit 230M is attached to the electrostatic latent image, a toner image of magenta (M) color is formed on the photosensitive drum 220, and the toner image is transferred onto the transfer belt 240.
In this case, the time at which the exposing unit 210 begins radiating light corresponding to the image information of magenta (M) color is adjusted in consideration of the transfer speed of the transfer belt 240, so that the front end of the toner image of the cyan (C) color formed previously on the transfer belt 240 is precisely consistent with the front end of the toner image of the magenta (M) color being transferred onto the transfer belt 240 from the photosensitive drum 220.
After toner images having cyan (C), magenta (M), yellow (Y), and black (K) colors are overlapped and formed on the transfer belt 240 by repeating the above operations for yellow (Y) and black (K) colors, the toner images are transferred and fused onto the piece of paper S to produce a color image.
FIG. 3 illustrates a color image forming system using the slider method. As shown in FIG. 3, four developing units 330C, 330M, 330Y, and 330K are arranged in the traveling direction of a photosensitive belt 320, and a cam 380 which selectively slides each of the developing units 330C, 330M, 330Y, and 330K out in a horizontal direction, one at a time, is provided.
The developing units 330C, 330M, 330Y, and 330K are initially placed so that a developing roller 331 is separated from the photosensitive belt 320 by an initial distance Di. Here, the initial distance Di is greater than a developing gap Dg (not shown) which allows toner attached to the developing roller 331 to be attached to the photosensitive belt 320. Thus, when each of the developing units 330C, 330M, 330Y, and 330K is separated from the photosensitive belt 320 by the initial distance Di, toner is not attached to the photosensitive belt 320 from the developing units 330C, 330M, 330Y, and 330K. However, when an image is formed, the cam 380 is rotated to slide a selected developing unit (230M in FIG. 3) toward the photosensitive belt 320 until the distance between the selected developing unit (230M in FIG. 3) and the photosensitive belt 320 is equal to the developing gap Dg. Thus, a development operation can be performed by only one selected developing unit at a time.
On the basis of the above configuration, the developing units 330C, 330M, 330Y, and 330K are selectively slid toward the photosensitive belt 320 by selectively operating the cam 380 so as to perform the development operation for each of cyan (C), magenta (M), yellow (Y), and black (K) colors, toner images of each color are formed on a transfer belt 340, are transferred onto the piece of paper S, and are fused onto the piece of paper S, thereby forming a color image.
However, in color image forming systems using a multi-path method and having either of the configurations shown in FIGS. 2 and 3, unselected developing units 230, 330 are separated from the photosensitive belt 320 or the photosensitive drum 220 by a distance greater than the developing gap Dg so that toners of the unselected developing units are prevented from attaching to the photosensitive drum 220 or the photosensitive belt 320 and contaminating the resultant color image. The developing units 230, 330 must be moved by rotating the turret 280 or operating the cam 380 so that only one selected developing unit 230, 330 at a time is placed a distance equal to the developing gap Dg away from the photosensitive belt 320 or the photosensitive drum 220. Thus, in order to rotate the turret 280 or operate the cam 380, an additional driving motor (not shown) must be provided. Otherwise, if an existing driver (not shown) of the color image forming system is used with a motor (not shown) to drive the photosensitive drum 220, a complicated apparatus for power conversion should be provided.
Hence, noise occurs when the turret 280 rotates or the cam 380 operates. Due to shock caused by the operation of the turret 280 or the cam 380, the lifespan of the driver (not shown) may be reduced. Moreover, such shock causes bands or jitter which reduces the quality of the resulting color image.